Hydraulic drive and steering systems for a vehicle

ABSTRACT

Front and rear wheeled units (13, 14) of vehicle are connected together for articulated steering. Wheels on at least one unit are mounted for swivelling relative to the respective unit to provide kingpin steering in combination with articulation steering. A first embodiment has front and rear kingpin steering and articulation steering performed manually and independently of each other. In a second embodiment, a steering integrator cooperates with actuators (162, 163, 212, 213, 236, 237) which control at least the articulation steering and kingpin steering of the front wheels (22, 23) to automatically integrate two types of steering so that a controlled relationship exists between an articulation angle of the front unit (13) with respect to the rear unit (14), and swivel angles of the front wheels (22, 23) with respect to the front unit (13). A manual steering control, e.g. a steering wheel (186), controls actuation of the steering integrator. Each wheel is powered by a hydraulic motor (32, 33, 44, 45) receiving pressurized fluid from a drive apparatus comprising a flow combiner and two flow restrictors connected in parallel with each other and communicating with outlets from the wheel motors. Bypass valves (114, 115) can be used to bypass the flow combiner and flow restrictors as needed in certain applications not requiring the flow combiner, to reduce generation of heat in the hydraulic fluid.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is the U.S. national stage application filed under 35USC 371 of international patent application No. PCT/CA95/00270, filedMay 4, 1995, which application is a continuation-in-part of U.S.application 08/237,910 filed May 4, 1994 which is now U.S. Pat. No.5,427,195, issued Jun. 27, 1995.

BACKGROUND OF THE INVENTION

The invention relates to a hydraulic drive apparatus and combinationsteering system for an industrial vehicle, in particular a four-wheeldrive tractor as used in rough terrain.

Hydraulic drive systems for tractors have been used for many years,particularly four-wheel hydrostatic drive systems which have particularapplications in terrain where there is poor traction, for example inagricultural, forestry, construction and mining applications. It iscommon for such vehicles to have hydraulic motors provided in thewheels, the motors being supplied with pressurized hydraulic fluid froma control system designed to reduce wheel slippage in poor tractionconditions. It is known to provide the two motors of an oppositelydisposed pair of wheels to receive pressurized hydraulic fluid from avariable displacement pump, the pump transmitting fluid to the wheelmotors disposed in parallel. A flow divider is commonly fitted upstreamof the wheel motors of the pair i.e. before motor inlets when drivenforwardly, to ensure that the fluid flow is divided essentially equallybetween the wheels. In this way, should a wheel of the pair losetraction and slip, thus tending to rotate at a higher speed than thewheel maintaining traction, the flow divider ensures that the slippingwheel does not receive much more fluid than the wheel maintainingtraction. Thus, power is still applied to the wheel maintaining tractionand increases the chance of the vehicle being able to proceed. However,if the vehicle descends a slope in the forward drive mode, and themotors are used to brake the vehicle, if a flow divider is locatedupstream of inlets of the motor, low pressure can be generated at theinlet of at least one motor, and this can cause severe damage to themotor due to cavitation. Chances of cavitation occurring can be reducedby use of anti-cavitation valves.

When such a vehicle is operated in reverse, the flow divider is nowlocated downstream from the wheel motors, and thus operates as a flowcombiner and thus attempts, in a similar manner, to ensure that bothwheels transmit essentially equal volume flows of fluid.

It is well known that use of a flow divider/combiner dissipates energy,and consequently heats the hydraulic fluid. If a vehicle is workingcontinuously, and the flow divider/combiner is operating continuously,continuous pressure loss generates considerable heat which requires anadequate heat sink, typically an air-cooled hydraulic fluid or oilcooler. In some circumstances, the capacity of the cooler isinsufficient and damage can result to the equipment from running onover-heated hydraulic fluid.

There are many patents relating to hydrostatic vehicle propulsionssystems, typical patents being as follows: U.S. Pat. Nos. 3,900,075(Chichester et al); 3,910,369 (Chichester et al); 3,916,625 (Holtkamp)4,244,184 (Baldauf et al) and 5,199,525 (Schueler).

Also, when a hydrostatic drive vehicle negotiates a turn, to avoid wheelscuffing, wheels on the outside of the turn rotate faster than wheels onthe inside of the turn, and thus the motor for the outside wheelrequires a greater flow of fluid therethrough than the motor for theinside wheel. The difference in fluid flow rates between inside andoutside wheels of a turn, hereinafter termed "flow differential",presents problems if a flow divider is provided in the inlet circuit,which attempts to maintain a constant flow through the wheelsirrespective of the turn. Various devices have been devised toaccommodate these problems and some of these devices are relativelycomplex and still result in scuffing of wheels during a tight turn.

For example, the above U.S. Pat. Nos. 3,900,075 and 3,910,369 discloseuse of a proportional flow divider controlling flow to a pair of wheels,in which the flow divider proportions or distributes flow between thewheels in proportion to angle of the turn but, as discussed above,continuous use of a flow divider can generate excessive heat.Furthermore, accuracy of flow proportions of some proportionaldividers/combiners is fairly poor, and thus the flow divider often canproportion flow improperly, causing excessive wheel scuffing duringturning.

There are two main ways of steering such vehicles, namely modifiedAckerman or kingpin steering, as used on most road vehicles, orarticulated steering, which is commonly used in rough terrain vehicles,e.g. industrial tractors, which, in the logging industry, are termed"skidders". In modified Ackerman steering, the front wheels swivel aboutrespective, generally vertical kingpins or equivalents which supportgenerally horizontal stub axes which journal the wheels. Whennegotiating a turn, the front wheels describe respective arcs, and theback wheels tend to "cut" the corner by describing arcs of smaller radiithan the arcs of the front wheels. Thus, when turning the vehicle, therear wheels tend to trail the front wheels laterally and thus requireadditional space on the inside of the turn.

The problem of trailing rear wheels is overcome by articulated steering,in which the vehicle has front and rear units connected together forarticulated steering about a generally vertical articulation axis. Eachunit has at least one pair of laterally spaced wheels, which arecommonly mounted on fixed beam axles with respect to the units, so thatthe wheels can rotate about horizontal axes with respect to each unit,but do not rotate about vertical axes. Such vehicles are rugged andrelatively successful in some applications, but have limitations inother applications on sensitive terrain. One problem is that, as thevehicle negotiates a turn, the front and rear units are articulated orinclined relative to each other at an angle, and the front and rearwheel pairs execute essentially identical arcs centred on a turn centre.In a normal turn, with no slippage, the rear wheels can followessentially exactly in the paths of the front wheels, and this candamage delicate or sensitive terrain, particularly if the wheels startto slip due to poor traction. U.S. Pat. No. 3,414,072 (Hodges Jr. et al)and U.S. Pat. No. 3,910,369 (Chichester) disclose such vehicles.

In contrast, U.S. Pat. No. 4,042,053 (Sieren et al) discloses afour-wheel drive tractor with articulated steering between front andrear units, but with the front wheels also mounted for kingpin steering.This patent discloses a mechanical, as opposed to a hydraulic, tractorwhich has direct, mechanically powered wheels and steering units, anduses transmission shafts and gears as opposed to hydraulically poweredmotors etc. and which is therefor relatively complex. This patent showsa vehicle with a combination of front wheel kingpin steering andarticulation steering which has several advantages. One of theadvantages recognized by the patentee is that for a given radius ofturn, the front wheels are inclined to the front unit at shallowerangles (i.e. less acute angles) than otherwise would be required for anormal kingpin steering. Similarly, Sieren et al recognized that therear unit is inclined to the front unit at a shallower angle than wouldotherwise be required for a normal articulation steering vehicle. Use ofshallower angles improves efficiency of power transfer to the wheels, aswell as tractive effort for any load pulled by the tractor. In addition,because the units operate at shallower angles to each other than normal,any mechanical transmission components are operating in improvedalignment with each other, which reduces wear and power consumption,thus improving life of the vehicle.

SUMMARY OF THE INVENTION

The invention provides a vehicle hydraulic drive system in which coolingloads of a hydrostatic drive system are reduced considerably when thehydrostatic drive is not required, as is found in some applications. Thecircuit of the drive system also prevents inlets of the hydraulic motorsbeing exposed to low pressure when the vehicle descends forwardly andthe motors are used to brake the vehicle. In addition, wheel scuffingduring a tight turn is reduced considerably by providing a simple andeffective means to accommodate the flow differential between the insideand outside wheels of the turn, without incurring high heat generation.

Also, the invention provides a steering apparatus of greater versatilitythan the prior art by providing three modes of steering for the vehicle,namely: articulation steering between front and rear units, and kingpinsteering for wheels mounted on the front unit, and also on the rearunit. The invention permits conventional operation of the kingpinsteering for the front wheels, with or without individual or independentcontrol of the articulation steering, and of the rear wheel kingpinsteering, which provides considerable versatility, but requires goodoperator skills for some types of manoeuvres. Alternatively, theinvention also provides a structure for automatically combining orintegrating at least two modes of steering, for example the frontkingpin steering and the articulation steering, thus simplifyingoperation of the vehicle for novice operators, with a slight loss ofversatility.

A vehicle hydraulic drive apparatus according to the invention comprisesa pressurized hydraulic fluid source, at least one pair of right handand left hand wheel motors, right hand and left hand bypass valves,right hand and left hand flow restrictors, and a flow combiner. Thehydraulic fluid source has a discharge which discharges pressurizedfluid under a relatively high pressure, and a return which scavengesfluid under a relatively low pressure. The right hand and left handwheel motors have respective fluid inlets and fluid outlets, each inletcommunicating with the discharge of the fluid source when the vehicle isdriven forwardly. The right hand and left hand bypass valves communicatewith the outlet of the respective motor and the return of thepressurized hydraulic fluid source when the vehicle is driven forwardly.The right hand and left hand flow restrictors communicate with theoutlet from the respective motor, and the return of the pressurizedfluid source when the vehicle is driven forwardly. The flow combiner hasright hand and left hand inlets communicating with the outlets from theright hand and left hand wheel motors respectively, and an outletcommunicating with the return of the fluid source when the vehicle isdriven forwardly. Preferably, the flow restrictors are in parallel withthe flow combiner and the bypass valves are in parallel with the flowcombiner and flow restrictors.

Another aspect of the invention provides a wheeled vehicle comprisingfront and rear units, an articulation steering actuator, front and rearkingpin steering actuators, and steering controls. The front and rearunits have right hand and left hand wheels as a pair, units beingconnected together to permit relative rotation therebetween about agenerally vertical articulation axis. Each wheel is mounted on itsrespective unit for kingpin steering to permit rotation of eachrespective wheel about a respective generally vertical swivel axisrelative to the respective unit. The articulation steering actuatorcooperates with the front and rear units to cause the said relativerotation to effect articulation steering as required. The front and rearkingpin steering actuators cooperate with each pair of front and rearwheels respectively to cause the rotation about the respective swivelaxes to effect the kingpin steering between the wheels of the respectiveunits as required. The steering controls control actuation of thearticulation actuator between the front and rear units, and controlactuation of the kingpin steering actuator with respect to at least oneor both of the units. The front kingpin steering control and rearkingpin steering control are operable independently of each other. Thearticulation steering control is operable independently of the controlfor controlling actuation of the kingpin steering actuator with respectto at least one or both of the units.

Another embodiment of the invention is a wheeled vehicle comprisingfront and rear units, an articulation steering actuator, a front kingpinsteering actuator, a steering integrator and a manual steering control.The front and rear units have right hand and left hand wheels as a pair,the units being connected together to permit relative rotationtherebetween about a generally vertical articulation axis. The frontwheels are mounted on the front unit for kingpin steering to permitrotation of each front wheel about a respective generally verticalswivel axis relative to the front unit. The articulation steeringactuator cooperates with the front and rear units to cause the saidrelative rotation to effect articulation steering as required. The frontkingpin steering actuator cooperates with the front wheels to cause therotation about the swivel axes to effect the said kingpin steeringbetween the wheels of the front unit as required. The steeringintegrator cooperates with the articulation steering actuator and thekingpin steering actuator to automatically integrate articulationsteering between the front and rear units and kingpin steering of thefront wheels. In this way, a controlled relationship exists between anarticulation angle of the front unit with respect to the rear unit, andswivel angles of the front wheels with respect to the front unit. Amanual steering control controls actuation of the steering integratorwhich in turn controls an articulation angle between the front and rearunits and respective swivel angles of the right hand and left hand frontwheels with respect to the front unit.

Preferably, the steering integrator comprises primary and secondarysteering signal apparatus. The primary steering signal apparatus isresponsive to the manual steering control and has a primary signaloutput. The secondary steering signal apparatus has an input connectableto the output of the primary steering signal apparatus and first andsecond outputs. The first output is transmitted to the articulationsteering actuator, and the second output is transmitted to the frontkingpin steering actuator. The first and second outputs have an outputsignal ratio which reflects the controlled proportional relationshipbetween the angle of the front units with respect to the rear unit andthe swivel angles of the front wheels with respect to the front unit.

Preferably, each front wheel has a steering assembly which includesrespective front tie rod arm extending therefrom and a tie rod assemblyconnecting together the tie rod arms. The tie rod assembly includes atie rod shortening structure for shortening effective length of the tierod assembly as the wheels are swivelled from straight line alignment.Preferably, the shortening structure of the tie rod assembly includesright hand and left hand tie rod portions and a steering bellcrank. Eachtie rod portion has respective outer and inner ends, the outer ends ofthe right hand and left hand tie rod portions being connected to the tierod arm of the right hand and left hand wheel assemblies respectively.The steering bellcrank interconnects inner ends of the right hand andleft hand tie rod portions, and is journalled for rotation in responseto movement of the wheels so that as the swivel angle of the front wheelwith respect to the front unit increases, effect length of the tie roddecreases.

A detailed disclosure following, related to drawings, describes twoembodiments of the apparatus according to the invention, which iscapable of expression in structure other than that particularlydescribed and illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified side elevation of a typical vehicle fitted with ahydraulic drive system and combination steering system according to theinvention,

FIG. 2 is a simplified schematic of the vehicle of FIG. 1 showing maincomponents of the hydraulic drive system for powering wheels of thevehicle, including connections to two control or logic blocks,

FIG. 3 is a simplified hydraulic schematic of one control block or logicblock of the hydraulic drive system of FIG. 2,

FIG. 4 is a simplified mechanical diagram and hydraulic schematic of afirst embodiment of a manual combination two or three-mode steeringapparatus according to the invention,

FIG. 5 is a simplified diagram showing wheels and main steeringcomponents of a vehicle according to the invention negotiating a tightturn when using the combination of three modes of steering with a fixedlength tie rod,

FIG. 6 is a simplified diagram similar to FIG. 5, the same vehicle beingshown in a crab-like straight line motion,

FIG. 7 is a simplified mechanical diagram and hydraulic schematicgenerally similar to FIG. 4, showing main steering components of asecond embodiment of a steering apparatus according to the invention,

FIG. 8 is a simplified diagram showing vehicle geometry of single modefront wheel kingpin or modified Ackerman steering without thearticulation steering, i.e. with the front and rear units locked in analigned position,

FIG. 9 is a diagram generally similar to FIG. 8 but showing a two modesteering combination of articulation steering and front wheel kingpinsteering, for a vehicle with a fixed length tie rod without swivel anglecorrection, and

FIG. 10 is a diagram which is generally similar to FIG. 9, but showing avehicle with variable length tie rod providing automatic swivel anglecorrection and a reduced turning radius of the vehicle, in a two modesteering combination of front wheel kingpin steering and articulationsteering,

FIG. 11 is a simplified mechanical diagram and hydraulic schematicshowing details of some steering components of the second embodiment ofFIG. 7, in the two mode steering combination of FIG. 10, and

FIG. 12 is a simplified fragmented mechanical diagram and hydraulicschematic generally similar to FIG. 7, but showing only a portion of afront unit of the vehicle, and illustrating a third embodiment of asteering apparatus according to the invention.

DETAILED DESCRIPTION FIGS. 1 and 2

A vehicle 10 according to the invention has a body 12 which comprisesfront and rear units 13 and 14, the units being connected together at anarticulation joint 16 to permit relative rotation therebetween about agenerally vertical articulation axis 18. The units 13 and 14 haverespective longitudinal unit axes 19 and 20 which intersect at thearticulation axis 18 as shown. The front unit has right and left handfront wheels 22 and 23 disposed as a front pair, and the rear unit hasright hand and left hand rear wheels 24 and 25 disposed as a rear pair,the pairs having similar track widths so that transverse spacingsbetween the wheels of each pair are equal. The front unit has anoperator cab 27, an engine 29 and a typical tool, e.g. an earth movingblade 30, as commonly used in site clearing or reclamation operations.The rear unit also has a typical tool, e.g. hinged boom 31 carrying agrapple, as used for handling trees. Clearly, many other different typesof tools could be substituted and fitted to the front and rear units,and alternative locations of the operator cab and engine arecontemplated. For many applications, but particularly for sitereclamation and silviculture, the vehicle preferably has a high groundclearance, particularly adjacent the articulation joint 16 to permit thevehicle to clear stumps, rocks and other debris on a reclaimed site.

Referring specifically to FIG. 2, the right hand and left hand frontwheels 22 and 23 of the front unit are powered by right hand and lefthand front wheel motors 32 and 33. The motors are reversible,piston-type hydraulic motors having an operating pressure range ofbetween 0 and 4800 psi, a suitable motor being manufactured by ReidvilleHydraulic and Mfg., Inc., a corporation of Connecticut, U.S.A. Themotors are integrated into the wheels, and are mounted on a front axlebeam 35, the front axle beam carrying kingpin steering units for thewheels as will be described with reference to FIG. 4. A front hydraulicpump 37 is connected through main conduits 40 and 41 to a front controlor logic block 39 which connects the motors 32 and 33 by a hydrauliccircuit which powers the motors and is described in greater detail withreference to FIG. 3. The front hydraulic pump 37 is a variabledisplacement, variable speed, piston pump having an output pressurewithin a range of between 600 and 5000 psi, a suitable pump beingmanufactured by Denison Hydraulics, Inc. , a corporation of Ohio, U.S.A.

Similarly, the rear unit 14 has right and left hand rear wheel motors 44and 45 driving the right hand and left hand rear wheels 24 and 25, themotors being similarly mounted on a rear axle beam 47. As will bedescribed with reference to FIG. 4, the rear wheels are similarlyprovided with kingpin steering, and thus the vehicle has four-wheelkingpin steering and articulation steering, which provides threeseparate modes of steering. The front unit also carries a rear hydraulicpump 49 which supplies fluid to, and receives fluid from, a rear controlor logic block 51 through main conduits 53 and 54 which is mounted onthe rear unit, and which, in turn, supplies fluid to the motors 44 and45 similarly to that front control block. The front and rear logicblocks 39 and 51 are essentially identical, and thus one only will bedescribed with reference to FIG. 3.

The vehicle includes an oil cooler 57 and an oil filter 58interconnected by a first cooler conduit 59. A second cooler conduit 62extends between the oil filter and is bifurcated to interconnect withthe front and rear pumps 37 and 49. A front cooler conduit 64 and a rearcooler conduit 66 interconnect the front and rear logic blocks 39 and 51with the front and rear pumps 37 and 49 respectively and in turn connectwith the conduit 62. The cooler conduits carry heated fluid from thelogic blocks to the pump casings through which the fluid flows prior topassing through the oil cooler 57 for cooling as is well known.Preferably, a pump interconnecting conduit 67 extends between the frontand rear pumps 37 and 49 and has a variable restrictor 69 to control anyfluid exchange between the pumps. The conduit 67 and restrictor 69 areparticularly applicable where the vehicle spends a lot of time in tightmanoeuvring situations, or where there is a wide variation of speedbetween front and rear wheel pairs. In other applications where thevehicle spends a lot of time on straight driving, the conduit 67 andrestrictor 69 can be omitted. The apparatus also includes many otherknown components which are conventionally found in hydrostatic vehicledrive units and are which not illustrated herein in any of the figures.Such components include variable speed devices, direction reversingdevices, fluid return lines, etc.

FIG. 3

The front logic block 39 is a hydraulic circuit and its relationship toother main components will be described, the front logic block beingshown within a rectangular broken outline, which corresponds to theoutline shown in FIG. 2.

The pump 37 serves as a pressurized hydraulic fluid source having adischarge 81 which discharges pressurized fluid under a relatively highpressure in the conduit 40 to the block 39, and a return 83 whichscavenges fluid from the circuit under a relatively low pressure in theconduit 41. Fluid from the pump is "processed" or controlled by thecontrol or logic block 39 which supplies fluid to, and returns fluidfrom, at least one pair of right hand and left hand wheel motors 32 and33 as shown.

In the following description, it is assumed that the vehicle is drivenforwardly and thus the pump 37 is driven in a forward direction, so thatfluid flows out of the discharge 81 in direction of an arrow 85 into thelogic block. The motor 32 has a fluid inlet and outlet 88 and 89respectively, and the motor 33 has a similar fluid inlet and outlet 90and 91 respectively. The fluid inlets 88 and 90 receive fluid fromconduits 94 and 95 respectively, which serve as inlet conduits extendingfrom the logic block 39. The outlets 89 and 91 discharge fluid intooutlet conduits 98 and 99 respectively which in turn pass into the logicblock.

The logic block has several components which function together tocontrol ratio of fluid flow volumes through the motors, when required,and, when needed, to send a controlled flow of hydraulic fluid to thecooler 57, (FIG. 2) as will be described. A main component of the logicblock is a flow combiner/divider unit 102 which operates as a flowcombiner when the vehicle operates in a forward mode, and as a flowdivider when the vehicle is in a reverse mode. When the unit 102 isfunctioning as a flow combiner, it has right hand and left hand inlets104 and 105 communicating with the conduits 98 and 99, and thus alsowith the outlets 89 and 91 from the right hand and left hand wheelmotors respectively. The flow combiner has an outlet 108 communicatingwith the main conduit 41, that is with the return 83 of the pressurizedfluid source. When the vehicle is driven forwardly, the flow combinerunites or combines outlet flows downstream from the motors inessentially equal proportions, i.e. in such a way that ratio of outletflow from one motor to outlet flow from another motor is within aclosely controlled ratio limit, e.g. about 5 per cent-6 per cent, as iswell known. Also as is well known, when the vehicle operates in reversemode, the flow combiner 102 functions as a flow divider, and thusproportions fluid flow upstream from the motors so that fluid flowsessentially equally into the outlet ports 89 and 91 of the motor 32 and33, which ports now serve as inlet ports.

The logic block 39 also includes a pair of right hand and left hand flowrestrictors or orifices 110 and 111, which are restricting orifices inconduits extending between the outlet conduits 98 and 99, and the mainconduit 41. It can be seen that the flow restrictors 110 and 111 areessentially in parallel with the flow combiner/divider 102, and thuscommunicate with the outlets 89 and 91 in the respective motor and thereturn 83 of the pressurized fluid source. The flow restrictors 110 and111 have metering bores of equal sizes which are selected to ensure thata speed differential can exist between the front wheels when the vehicleis negotiating a turn to essentially prevent scuffing of the wheels, aswill be described. As is well known, when a vehicle negotiates a turn,the outside wheel rotates faster than the inside wheel, consequently theoutside wheel hydraulic motor passes a greater volume of fluid than theinside wheel motor in proportion to radius of the turn, "track" size ortransverse width between the wheels of the pair and other factors, andis defined herein "flow differential". The flow restrictors 110 and 111have a size sufficient to enable this flow differential to exist betweenthe wheels describing a turn of minimum radius with negligible scuffing.It has been found that ratio of maximum flow through one flow restrictorto maximum flow through the flow combiner is about 1:10 and thisprovides a sufficient flow differential for many applications. This flowdifferential would be very much less if the flow combiner 102 was usedby itself i.e. without the flow restrictors, because a flow combinerensures that flow from the wheels is held within much smaller tightlycontrolled limits and this would cause scuffing while turning.

The logic block 39 further includes right hand and left hand bypassvalves 114 and 115 which communicate with the motor outlets 89 and 91through valve conduits 118 and 119 respectively. The conduits 118 and119 extend from the valves to a common conduit 122 which in turnconnects with the main conduit 41 to communicate with the return 83 ofthe pressurized fluid source. The bypass valves 114 and 115 are two-way,two-position, normally-closed solenoid valves which are connectedelectrically by means, not shown, to a switch in the operator's cab toenable the operator to energise both the valves as required. As shown,the valves are in an de-energized state and thus the conduits 118 and119 are closed and all fluid returning from the motors 32 and 33 mustpass through either the flow combiner 102, or one of the restrictors 110and 111. It can be seen that the flow restrictors 110 and 111 are inparallel with the flow combiner 102, and the bypass valves 114 and 115are in parallel with the flow combiner and flow restrictors. When thevalves 114 and 115 are opened, the conduits 118 and 119 have anessentially negligible resistance to flow when compared to therestrictors or the flow combiner, and thus nearly all fluid would flowpreferentially through the conduits 118 and 119. When the vehicle goesinto reverse, clearly the flow combiner functions as a flow divider andthus can accommodate reverse flow therethrough. Similarly, the bypassvalves and flow restrictors can accommodate reverse flow therethroughwhen the vehicle operates in reverse mode.

The front logic block 39 further includes a hot oil shuttle valve 125having a pair of inlet ports 127 and 128 which communicate with conduits123 and 124 extending from the discharge 81 and the return 83respectively of the pump 37, and thus the ports 127 and 128 are termedherein a discharge connecting port 127 and a return connecting port 128respectively. The valve 125 also has an outlet port 130 communicatingwith an outlet line 132 which connects to the line 64 (FIG. 2) which,via the casings of the pumps, eventually communicates with the cooler 57(FIG. 2). The port 130 also communicates with a gauge line 137 leadingto a pressure gauge 138 for monitoring pressure.

The hot oil shuttle valve 125 is a three-way, three-position,pilot-pressure operated valve in which pilot lines 140 and 141 controlposition of the valve by being responsive to a pressure differentialbetween the conduits 123 and 124 respectively. The valve 125 functionsto divert fluid heated during operation of the vehicle to the cooler 57by detecting a sufficient pressure difference between the appropriatepilot lines. For example, if pressure difference between the pilot lines140 and 141 indicates that the pressure in the line 124 is very muchless than that in the conduit 123, e.g. about 200 psi less, this pilotpressure difference actuates the valve 125 to connect the line 124 withthe outlet port 130 to transmit fluid at lower pressure to the coolerthrough the lines 132, 64, etc. Thus the cooling valve 125 is responsiveto pressure difference between conduits 123 and 124 extending from thedischarge and return of the fluid source, and the outlet port 130communicates with the cooler as required so as to direct fluid at thelowest pressure to the cooler. The pilot line with lowest pressurereflects "spent" fluid which is to be diverted to the cooler.

Pressure in the outlet line 132 is controlled by a pressure releasevalve 135 which similarly has a pilot line 145 which directs thepressure signal from the outlet port 130 through the line 132 so thatthe valve 135 controls back pressure on the shuttle valve to ensure aback pressure in the system.

The rear logic block 51 is generally similar to the front logic blockand is essentially hydraulically independent therefrom, apart from theoptional limited fluid connection in the interconnecting conduit 67,which interconnects the front and rear pumps 37 and 49, and also throughthe cooler conduits 62, 64 and 66 as seen in FIG. 2. Thus, loss oftraction for one of the front wheels has a negligible effect onoperation of the rear wheels and vice versa. Similarly to thatpreviously described with reference to that logic block 39, the block 51has valves equivalent to the valves 125 and 135 of FIG. 3 to pump heatedfluid from the logic block 51 to the conduit 66 (FIG. 2), and to thecooler.

Thus, it can be seen when the rear logic block and associated componentsare included in the total fluid circuit for the apparatus, the apparatusincludes an additional pressurized hydraulic fluid source, namely therear pump 49 of FIG. 2, the right hand and left hand wheel motors 44 and45 of the rear unit, and additional right hand and left hand bypassvalves and flow restrictors, not shown, which are equivalent to thevalves 114 and 115 and flow restrictors 110 and 111 of FIG. 3.Similarly, an additional flow combiner, not shown, which is equivalentto the flow combiner 102 is provided for the rear logic block 51 whichwill function essentially identically to the block 39.

FIG. 4

A first embodiment 151 of a three-mode steering apparatus according tothe invention will now be described. The right hand and left hand frontwheels 22 and 23 and associated motors 32 and 33 (not shown) are mountedon the front axle beam 35 using conventional kingpin or modifiedAckerman steering structure. Similarly, the rear wheels 24 and 25 andassociated motors 44 and 45 (not shown) are mounted on the axle beam 47with generally similar steering structure, and thus only the front wheelunit will be described in detail. Thus, each wheel is mounted on itsrespective unit for kingpin steering to permit rotation of eachrespective wheel about a respective generally vertical swivel axisrelative to the respective unit. The swivel axes for the right hand andleft hand front wheels are designated 154 and 155 and conventionalkingpins or equivalents, suspension members, and bearings etc. areprovided but are not shown or described in detail. For completeness,some main components will be described as follows.

The right hand wheel is mounted for rotation about a generallyhorizontal stub axle 158, shown in broken outline, which can be swungabout the swivel axis 154 as is well known. The axle 158 also carriesthe hydraulic motor 32 (see FIG. 2), and thus the wheel and motor forman integral unit which is swung with respect to the axle beam 35 by useof a tie rod arm 160 and an actuator arm 162 which extend from thekingpin or equivalent and are inclined to the stub axle as shown.Similarly, the left hand wheel 23 can be swung and has a left hand tierod arm 165 and a left hand actuator arm 166 inclined similarly asmirror images to the arms 160 and 162. Axes 167 and 168 of the tie rodarms 160 and 165 respectively are shown projected rearwardly andintersect at a intersection 164 which coincides with a rear axle axis210 passing through the swivel axes 208 and 209 of the rear wheels. Inother words, the tie rod arm axes projected rearwardly intersect at therear axle axis 210 when the wheels are aligned straight, which followsconventional two-wheel kingpin steering design. A tie rod 169 is hingedto and extends transversely between the tie rod arms 160 and 165 tocouple the front wheels together to coordinate swivelling of the frontwheels and to effect modified Ackerman steering as is well known.

The embodiment 151 further includes right hand and left hand hydraulickingpin steering cylinders 172 and 173 or steering actuators whichextend between the respective actuator arms 162 and 166 and a portion ofthe front unit. The steering cylinder 172 has an extension port 177 anda retraction port 178 in which the kingpin cylinder extends or retractswhen sufficient positive pressure is applied to the extension orretraction port respectively. Similarly, the left hand kingpin steeringcylinder 173 has extension and retraction ports 181 and 182respectively.

The operator cab 27 houses a steering wheel 186 mounted on a rotatablesteering column 187 which is coupled to a conventional power steeringvalve and pump unit 189. The unit 189 is a conventional power steeringunit comprising a four-way proportioning control valve having first andsecond signal ports 191 and 192, and an inlet port 185 which receiveshydraulic fluid under pressure from a first steering pump 184. As in allsteering valves herein, the signal ports are bidirectional, i.e. fluidflows inwards or outwards depending on direction of turn, and anundesignated fourth port returns fluid to a sump, not shown. Right handextension and retraction conduits 198 and 199 extend between the ports192 and 191, respectively. Similarly, left hand extension and retractionconduits 202 and 203 extend between the ports 191 and 192 respectively.Rotation of the steering wheel 186 changes flow through the valve 189which directs fluid under pressure to appropriate chambers of thecylinders 172 and 173, so as to supply fluid in such a direction as toextend one cylinder and to retract the other, causing concurrentswivelling of the front wheels about the respective axes to effectkingpin steering. The tie rod 169 couples the wheels together tocoordinate swivelling and thus, in theory, only one kingpin steeringcylinder is required. However, following common practice, to reduceloads on the tie rod and to enable smaller diameter cylinders to beused, it is preferable to have two kingpin cylinders 172 and 173 whichextend and retract simultaneously and thus work in concert with eachother and in combination with the tie rod 169. Structure associated withthe front wheel steering is essentially conventional, and provides frontkingpin steering to control swivel angle of the front wheels, and thusfurther description is deemed unnecessary. As is also common practice,the unit 189 incorporates a pump to permit steering if the hydraulicpressure from the pump 184 becomes unusable.

The rear wheels 24 and 25 are also mounted for kingpin steering withrespect to the rear axle beam 47, and thus are mounted to swivel aboutgenerally vertical swivel axes 208 and 209 respectively. Similarly tothe front wheels, the rear wheels have tie rod arms and actuator armsextending as shown, which cooperate with a transversely extending tierod 211, and right hand and left hand kingpin steering cylinders orsteering actuators 212 and 213 respectively. Similarly to the frontwheel tie rod arms, axes of the rear wheel tie rod arms associated withthe rear wheels are inclined equally to the longitudinal vehicle axiswhen the wheels are aligned for straight line travel and when projectedforwardly intersect at an axis of the front wheel axle.

In contrast to the front wheel steering which uses the power steeringunit 189, the rear wheel steering is controlled by an electricallyactuated steering valve 216. The valve 216 is a four-way,three-position, closed-centre, solenoid-operated directional valve whichhas a first signal port 218 communicating with correspondingundesignated retraction and is extension ports in the cylinders 212 and213, and a second signal port 219 communicating with correspondingundesignated extension and retraction ports in the cylinders 212 and213. The valve 216 has an inlet port 223 which receives pressurizedfluid under pressure through a fluid supply line 224, which in turnreceives fluid under pressure from a second hydraulic steering pump 226which is generally equivalent to the pump 184. The valve 216 iscontrolled remotely from the operator cab through electrical wires, notshown, which connect to a simple manually-actuated two-way switch in thecab which controls the valve 216 to direct fluid as required to oppositesides of pistons in the cylinders 212 and 213 to simultaneously extendand retract the cylinders to effect rear wheel steering. This providestwo similar modes of steering which has advantages as will be describedwith reference to FIGS. 5 and 6. Thus, the rear kingpin steering controlfor the rear unit comprises a rear steering actuator unit which is adirectional control valve controlling angle of the rear wheels withrespect to the rear unit.

The pump 226 also supplies fluid under pressure to an auxiliary powertake-off 228 which can be used to power equipment on a vehicle such asthe blade 30 and the boom 31 as shown in FIG. 1. The blade and/or boomis controlled through an auxiliary valve unit 230 which receiveshydraulic fluid under pressure from a line 232 connecting with the line224.

The three-mode steering apparatus also includes articulation steeringabout the articulation joint 16 which is controlled by a manuallyactuated articulation steering valve 225 also controlled in the cab. Thevalve 225 is a four-way, three-position, closed-centre,solenoid-operated directional valve which can be generally similar tothe valve 216 and which receives pressurized fluid through the line 232from the pump 226 similarly to the valve 230. The front and rear unitsare swivelled about the axis 18 by right hand and left hand hydraulicarticulation steering cylinders or articulation actuators 236 and 237.The cylinders 236 and 237 are parallel to each other, extend between thefront and rear units, and are spaced on opposite sides of thearticulation joint with respect to the longitudinal axis of the vehicle.The cylinders are essentially similar, and thus only the right handcylinder will be described. The cylinder 236 has extension andretraction ports 240 and 241 respectively which cooperate with first andsecond signal ports 242 and 243 respectively of the valve 225. The lefthand articulation actuating cylinder 237 has similar extension andretraction ports which similarly cooperate with the valve 225. Clearly,extension of one cylinder and simultaneous retraction of the othercylinder causes the units 13 and 14 to swivel about the axis 18 as iscommon practise. Thus, the vehicle has an articulation steering controlwhich is a directional flow control valve i.e. the valve 225, whichinterchanges connectors between the articulation cylinders forcontrolling the articulation angle between the front and rear units.

It can be seen that the actuation of kingpin steering of the rear wheelsis controlled with the valve 216, and actuation of the articulationsteering is controlled through the valve 225, which valves are similarlycontrolled remotely by respective electrical switches in the cab. Bothtypes of valves permit control of flow direction to and from theappropriate cylinders, and thus steering angles intermediate of fulllock-to-lock positions for both the swivel angles and the articulationangle can be attained. Usually, the resulting steering angle for eitherthe rear steering wheels, or the articulation joint is proportional tothe time that the particular manual control switch is actuated. In otherwords, the longer the operator maintains a particular valve "on", i.e.being actuated, the greater the change in the steering angle.

Similarly to conventional vehicles, the present invention is shown withtwo articulation steering cylinders 236 and 237. In some vehicles, asingle articulation steering actuator which cooperates with the frontand rear units to cause the said relative rotation can be substitutedfor the two steering cylinders as shown.

Clearly, there are three independent main controls in the cab for thethree modes of steering as described. One control, e.g. a switch for thevalve 225, controls actuation of the articulation steering actuatorextending between the front and rear units. There is also the steeringwheel 186 and the valve 216 for controlling actuation of the kingpinsteering actuators with respect to at least one or both of the unitsi.e. the front, or the front and rear wheels.

In summary, it can be seen that front and rear kingpin steeringactuators cooperate with each pair of the front and rear wheelsrespectively to cause the rotation about the respective vertical swivelaxes to effect independent and proportional kingpin steering between thewheels of the respective units as required. Clearly, the articulationsteering control is also operable independently of the control forcontrolling actuation of the kingpin steering actuators with respect toat least one or both of the units.

OPERATION

Operation of the apparatus follows generally that of similar prior artapparatus, with the exception that operation of this apparatus has achoice of three steering modes, namely front wheel kingpin steering,rear wheel kingpin steering, and articulation steering or a combinationthereof. This results in a more manoeuvrable vehicle which requires lesslateral space for passing between obstructions in congested areas. Theoperator steers the vehicles using the steering wheel in a relativelyconventional manner for single mode front wheel kingpin steering, andselects articulated steering and/or rear wheel kingpin steering modeswhere appropriate, thus incorporating the two or three modes of thesteering when required.

FIG. 5

The vehicle 10 is shown negotiating a turn of minimum radius 251 about aturn centre 253, and this requires front and rear wheel kingpin steeringand articulation steering. For convenience of explanation, a transverseaxis 255 is shown passing through the turn centre and the articulationaxis 18, the axis 18 passing along a circular arc 256 in direction of anarrow as shown. To negotiate this turn, the axis 19 of the front unitand the axis 20 of the rear unit are inclined to each other at an angle258 as shown. The front wheels 22 and 23 are rotating about front wheelaxes 260 and 261 which, when projected inwardly, intersect the axis 255at intersection points 264 and 265 respectively. Similarly, the rearwheels 24 and 25 are rotating about rear wheel axes 268 and 269respectively which intersect the axis 255 also at the points 264 and 265respectively. The intersection points 264 and 265 of the axes of thewheels on the outside and inside of the turn respectively are spacedapart, indicating that some scuffing of the wheels will take place inthis mode of steering. The said wheel scuffing can be reduced if atleast one swivel angle of the front wheels is "corrected" as will bedescribed with reference to FIGS. 7-11.

It can be seen that the rear wheels follow closely behind the frontwheels and this is not a problem on a hard surface. However, if desired,the front and/or rear steering can be adjusted so that the rear wheelstrace an arc different from the arc traced out by the front wheels. Inthis way, the rear wheels do not pass over paths 262 and 263 of thefront wheels 22 and 23 which is advantageous when operating in sensitiveterrain, as it reduces terrain damage. In addition, the rear unit can bemade to follow closely behind the front unit, thus requiring lesslateral spacing between obstructions. This contrasts with conventionalkingpin steering on a rigid chassis, in which, during a tight turn, therear wheels "cut off" the corner while negotiating the turn. The"cutting off" by the rear wheels occurring in conventional steeringrequires additional space between obstructions, which would not berequired in the present invention. Even when the present invention iscontrasted with the prior art vehicle having a combination ofarticulated steering and front wheel kingpin steering only, the presentinvention requires less space for turning when the rear wheels areswivelled in an equal but opposite direction to the front wheels, andthe body is actuated in the same direction as the front wheels as shown.

While the description above refers to FIG. 5 in which three modes ofsteering are shown, clearly it would be also applicable for applicationsin which only one or two steering modes are used, in which cases theradius of the turn would be larger and the flow differential (as definedwith reference to FIG. 5) would be correspondingly less.

FIG. 6

The vehicle is shown travelling in a straight line in direction of theundesignated arrow, with the unit axes 19 and 20 aligned with each otherand the wheels of each unit being swivelled equally to each other to beset in a crab-like mode. Thus paths 272 and 273 of the front wheels 22and 23 are not traversed by the rear wheels 24 and 25, which producerespective paths 274 and 275 which are spaced laterally from the paths272 and 273 as shown. Thus the rear wheels do not traverse terrainalready traversed by the front wheels, thus reducing terrain impact.

FIGS. 2 and 3

The front and rear logic blocks 39 and 51 operate generally similarly,and thus description of the hydraulic circuit will be limited to thatshown in FIG. 3, except where there is cooperation between the two logicblocks which are shown only in FIG. 2.

Referring to FIG. 3, during forward travel the front hydraulic pump 37discharges pressurized fluid (in "forward direction") shown by the arrow85 along the main conduit 40 which supplies fluid to the inlets of theright hand and left hand motors 32 and 33 respectively. Several types ofoperation on different types of surfaces will be described as follows.

Driving in an essentially straight line in good traction conditions(i.e. on a non-slipping surface) is a very common mode of operation insome applications. In this type of application there is negligibleslippage between the wheels so that the wheels rotate at essentiallyequal speeds and essentially equal fluid volumes pass through the motorsto be discharged into the outlet conduits 98 and 99 respectively. If theoperator knows that there will be little tendency for the wheels toslip, the bypass valves 114 and 115 can be energized, so that the valvesare opened and essentially equal volumes of fluid from the motors passprimarily through the conduits 118 and 119, i.e. most flow bypasses thehigher resistance of the flow combiner/divider 102 and the orifices 110and 111. Because most of the fluid from the motors 32 and 33 passesthrough the valves 114 and 115, relatively little flows through theorifices 110 and 111 and essentially none flows through the flowcombiner/divider 102 which therefore operates at a very low rate, andthus relatively little heat is generated in the fluid which is returnedto the pump.

When the operator anticipates that the vehicle will be required tonegotiate a series of curves in good traction conditions, the valves 114and 115 can be energized so as to be opened similarly to straight linetravel. Thus, the fluid bypasses the flow combiner and orifices andflows through the valves 114 and 115 in associated conduits as before.However, when negotiating a turn there is the flow differential aspreviously defined between flow through the motors driving the wheels onthe outside of the turn, and motors on the inside of the turn. Becausethe valves 114 and 115 are open and the conduits 118 and 119 offerlittle resistance to flow, most of the fluid flowing through the motorspasses through the conduits 118 and 119. The conduits have sufficientcapacity to accommodate most if not all of the flow differential betweenthe motors of the wheels of the inside and the outside of the turn, andthus there is negligible wheel scuffing during turning and negligibleheat generated by the orifices and flow combiner which are effectivelybypassed by the conduits 118 and 199.

In contrast, when the vehicle is driven in a straight line in poortraction conditions, (e.g. on a slippy surface) it is important thatboth wheels are controlled to rotate at approximately the same speed,and consequently the valves 114 and 115 are maintained closed so thatall flow from the motors passes through one of the orifices or the flowcombiner. The flow combiner 102 attempts to maintain the output flowsgenerally equal to each other, and any minor differences in flowspassing through the orifices 110 or 111 is immaterial and essentiallyindependent of the flow combiner. Wheel slippage can occur duringstraight line travel, but in general any terrain damage caused by suchslippage is usually negligible. Thus, if one wheel starts to slip whenthe vehicle is travelling in a straight line, if there were no flowcombiner, the flow through the motor of that wheel would increase,thereby further compounding the speed difference. However, when there isa flow combiner, the flow through the motor of the slipping wheel doesnot increase appreciably but only at a rate dependent on the flowcombiner and the orifices. Thus one of the wheels can slip, but theslippage is limited by the orifices and flow combiner and thus the wheeldoes not slip in an uncontrollable manner, and thus causes negligibledamage to the terrain.

When negotiating a turn in poor traction conditions, the valves 114 and115 are again closed and thus all flow from the motors passes throughone of the orifices or the flow combiner. For example, when negotiatinga left hand turn as shown in FIG. 5, the right hand wheels 22 and 24, onthe outside of the turn, rotate at higher speeds than the left handwheels 23 and 25 on the inside of the turn. Consequently, volume of flowpassing through the right hand motors 32 and 44 is greater than thevolume passing through the left hand motors 33 and 45. Thus, as shown inFIG. 3, flow through the conduit 98 is greater than flow through theconduit 99, and the difference in flow is accommodated by the orifices110 and 111 because the flow combiner attempts to match the flows. Theorifices can pass the flow differential to a maximum of about 10 percent as previously described, and thus speed differential between thewheels in a turn of minimum radius is approximately 10 per cent,although it is also dependent on speed of the vehicle. The approximate10 per cent speed differential between the wheels is usually sufficientto permit the tightest turn to be negotiated with negligible scuffingbetween the wheels. If the outside wheel starts to slip, the flowthrough that motor would attempt to increase further, but any increasewould again be restricted by the orifices and the flow combiner,improving transfer of power to the non-slipping inside wheel.

Size of the orifices of the restrictors 110 and 111 is found by trialand error by visually monitoring wheel scuffing while executing tightturns and adjusting size of the orifice accordingly to reduce wheelscuffing to a minimum. It has been found that wheel scuffing can beeasily detected on a delicate surface and thus it is advantageous toperform such testing on a delicate surface, that is a surface which iseasily recognizable to be damaged by scuffing. From a practicalstandpoint, the orifices are made to be interchangeable and are testedby initially using a size that is too small and increasing the sizeincrementally to determine the minimum size that can accommodate themaximum flow differential. Preferably, size of the orifice can be easilyadjusted externally of the apparatus by locating the orifices in aconvenient and accessible place to enable the operator to make theadjustment. This would be particularly necessary if the vehicle operatedat different times with different size wheels. It is added that size ofthe orifices should be selected to accommodate maximum flow differentialthat will occur with a turn of minimum radius, that is as shown in FIG.5, with the vehicle operating with a combination of three modes ofsteering. Clearly, if the vehicle executes turns of a greater radius,the flow differential will be less but the orifices will function in asimilar manner.

When the vehicle descends a grade in the forward direction in anysurface condition, the flow combiner is operative by closing the bypassvalves 114 and 115, thus ensuring that most of the flow passes throughthe flow combiner. As the flow combiner receives fluid from the outletsof the motors, when weight of the vehicle dominates and "drives" themotors which act as pumps and transmit pressurized fluid to the pump 37.The pump 37 in turn now acts as a motor and attempts to drive the engine29 as is well known. The pump 37 can still maintain an output pressurein the inlets 88 and 90 of the motors because there is no restriction atthe inlets that occurs in some prior art apparatus in which a flowdivider is commonly fitted upstream from the motors. In prior artapparatus fitted with a flow divider upstream from the motors, whendescending a grade, the inlet of the motors can be exposed to lowpressure due to flow restriction caused by the flow divider, which canproduce cavitation difficulties. Thus, when descending a grade with themotors acting as brakes, in the present invention, the motors operateunder higher pressure than in some prior art vehicles, thus avoidingprior art cavitation problems. Fluid restriction at outlets of themotors 32 and 33 caused by the flow combiner 102 also increases pressureslightly at the outlets of the motors but this has a negligible effect.

From the above, it can be seen that the bypass valves 114 and 115 arenormally maintained closed to permit full use of the flow combiner andorifices for operating conditions such as excessive turning, descendinggrades, and/or use on poor traction surfaces, in which conditions thevehicle is usually travelling relatively slowly. When travellingrelatively slowly, in general fluid heating problems are reduced becauseflow through the flow divider is reduced. However, when the vehicle istravelling on good traction surfaces in a straight line, for examplebeing ferried from one site to another, typically the vehicle operatesat relatively high speed, and at such times the bypass valves areactivated, so as to bypass the flow combiner. In this latter situation,if a flow combiner were being used, it would be handling a considerablefluid flow, and thus would likely cause severe heating of the fluid, andthus it can be seen that the ability to bypass the flow combiner inrelatively high speed operation of the vehicle provides a considerableadvantage by reducing cooling demands.

ALTERNATIVES

The first embodiment of the invention has three independent modes ofsteering and only one of these is easily and accurately controlled bythe operator, that is the front wheel kingpin steering as controlledthrough the steering wheel in the normal manner. The articulationsteering between the front and rear units, and the rear wheel kingpinsteering both require separate individual manual control, which can bedifficult to operate for a novice operator, or when negotiating aconstricted area while performing other tasks with the apparatus carriedby the vehicle. In these circumstances it is more convenient to have anapparatus which automatically combines two or even three modes ofsteering, preferably all modes being operable through a single operatorcontrol, for example, the steering wheel. Two examples of a vehiclecombining two modes of steering are described with reference to FIGS. 7through 12.

FIG. 7

A second embodiment vehicle 280 is shown fitted with a first alternativecombination steering apparatus 282 which is a combination of front wheelkingpin steering and articulation steering, termed integrated steering,which uses automatic compensation of the kingpin steering swivel angles,thus reducing demands on the operator as will be described. Much of thevehicle 280 is the same as the vehicle 10 of FIGS. 1 through 4 andconsequently components which are essentially identical are designatedwith identical numerical references. For simplicity, only the majordifferences between the two types of steering apparatus will bedescribed.

The two main differences between the apparatus 282 of FIG. 7 relate tosubstitution of an alternative power steering valve and pump unit 285and related valves for the unit 189 of FIG. 4, and a substitute of analternative tie rod assembly 287 for the tie rod 169 of FIG. 4.

The steering valve unit 285 is a conventional four-way power steeringvalve and pump unit which can be similar to the unit 189 of FIG. 4 andis controlled by rotation of the column 187 to serve as a primarysteering signal apparatus. The valve unit 285 is responsive to rotationof the manual steering control and has first and second primary signalports 291 and 292 which serve as a primary signal output and return asappropriate, and an inlet port 290 which receives pressurized fluid fromthe pump 226. The steering apparatus 282 also includes first and secondproportioning valves 295 and 296 which are conventional three-wayproportioning valves, each valve having three bidirectional ports asfollows. The three ports comprise a combined port and two split ports,such that flow volume through the combined port equals sum of flowvolumes through the two split ports.

The valves 295 and 296 have combined ports 297 and 298 respectivelywhich communicate with the first and second primary signal ports 291 and292 respectively to receive signals therefrom and to return fluidthereto. The first proportioning valve 295 also has a first front splitport 299 which communicates with the actuators 172 and 173 through firstconduits 301 and 302 respectively. Similarly, the second proportioningvalve 296 also has a second front split port 308 which communicates withthe actuators 172 and 173 through second conduits 310 and 311respectively. Thus the first front split port 299 communicates with theretraction and extension ports 178 and 181 of the cylinders 172 and 173respectively, and the second front split port 308 communicates with theextension and retraction ports 177 and 182 of the cylinders 172 and 173.This coupling ensures concurrent extension and retraction or vice versaof the cylinders 172 and 173 for controlling angles of the front wheels.

The first proportioning valve 295 has a first rear split port 305 whichcommunicates through a second conduit 306 with extension and retractionports 314 and 315 of the articulation actuators 236 and 237respectively. Similarly, the second proportioning valve 296 has a secondrear split port 312 which communicates through a second conduit 316 withextension and retraction ports 317 and 318 of the articulation cylinders237 and 236 as shown.

Thus, it can be seen that the first and second proportioning valves 295and 296 are a portion of a second steering signal apparatus whichcomprises first and second proportioning units, namely the valves 295and 296, having first and second combined ports communicating with thefirst and second primary signal ports of the primary steering signalapparatus, with each proportioning unit also having a front split portand a rear split port. The front split port of each proportioning unitcommunicates with the front kingpin steering actuator, and the rearoutput of each proportioning unit communicates with the articulationsteering actuator.

For convenience of discussion herein, the signal communication betweenthe steering valve unit 285 and the proportioning valves 295 and 296 canbe considered to be that of primary and secondary steering signals, thedirection of high pressure signal flow being from "outputs" of the valveunit 285 to "inputs" of the proportioning valves 295 and 296, whichvalves have corresponding "outputs" to respective cylinders. Forsimplicity, the corresponding low pressure return in a reverse directionthrough the valves is ignored in the following discussion. The steeringvalve unit 285 is shown as having two primary signal ports 291 and 292,and thus this functions as a primary steering signal apparatus which isresponsive to the manual steering control and has a primary signaloutput, namely the ports 291 and 292. The two proportioning valves 295and 296 serve as secondary signal steering apparatus having an input,namely the combined ports 297 and 298, which are connectable to theprimary signal output of the primary steering signal apparatus. Thesecondary steering signal apparatus has a first output, namely the rearsplit port 305 and 312 which are transmitted to the articulationsteering actuator, and a second output, namely the front split ports 299and 308, which are transmitted to the front kingpin steering actuators.The first and second outputs have an output signal ratio which reflectsthe controlled proportional relationship between the angle of the frontunit with respect to the rear unit and the swivel angles of the frontwheels with respect to the front unit.

The first and second proportioning valves 295 and 296 are essentiallyidentical to each other and thus only the first valve will be described,as follows, and also with reference to FIG. 11. For a turn to the left,the valve 295 proportions or divides input flow passing through thecombined port 297 between the front split port 299 and the rear splitport 305 in a fixed volume ratio, i.e. the previously defined outputsignal ratio, so that appropriate volumes of fluid are fed to theappropriate cylinders. The valve 296 accepts return flow in a reversedirection in similar proportions from the appropriate cylinders whichflow is returned back to the steering valve 285. The total proportion ofthe flow volume between the pair of articulation cylinders 263 and 267and the pair of king pin steering cylinders 172 and 173 as controlled bythe output signal ratio of the proportioning valves is termed "steeringfluid ratio". This ratio is critical and is essentially constant for allfluid flows between full left lock and full right lock and is based onthe flow ratio necessary to achieve corresponding proportionaldisplacements of the articulation actuating cylinders with respect tothe kingpin steering cylinders. In other words, the output signal ratioof the secondary steering signal apparatus is equal to the steeringfluid ratio and is defined by fluid volume flow with respect to thearticulation steering actuator and fluid volume flow with respect to thefront kingpin steering actuator. Also, ratio of total volume flow withrespect to the rear split ports and total volume flow with respect tothe front split ports defines the steering fluid ratio, which controlsrelative actuation of the articulation steering actuators and thekingpin steering actuators.

The output signal ratio as described above is dependent on manyvariables which relate to physical characteristics of the vehicleitself. Such characteristics include wheel base of the vehicle, that islongitudinal spacing between wheels on the same side of the vehicle,track of the wheels, that is transverse spacing between the wheels of aunit, relative lengths of the actuating arms 162, 166 and position ofthe kingpin steering cylinders, length of the tie rod arms 160, 165lateral spacing of the articulation cylinders 236, 237 from thearticulation axis 18, relative displacements of the kingpin steeringcylinders 172, 173 and the articulation cylinders 236 and 237 and othervariables as is known. However, for most vehicles, all the abovecharacteristics are fixed, and the steering fluid ratio is determined bygeometrical considerations which are briefly discussed with respect toFIGS. 9 and 10, and partially recited in Table 1 which follows, and alsodescribed with respect to the alternative tie rod assembly 287 asfollows.

The alternative tie rod assembly 287 comprises right hand and left handtie rod portions 320 and 321 respectively and a steering bellcrank 323journalled for rotation with respect to the front unit about a bellcrankaxis 325. The tie rod portions are of equal length and the axis 325intersects the unit axis 19, i.e. the bellcrank is symmetricallylocated. The tie rod portion 320 has an outer end 327 connected to thetie rod arm 160 of the right hand wheel and an inner end 328 connectedto a right hand arm 330 of the bellcrank 323. Similarly, the left handtie rod portion 321 is connected to the left hand tie rod arm 165 and aleft hand arm 331 of the bellcrank. When the wheels are alignedsymmetrically with respect to the front unit for straight line travel,the bellcrank 325 is disposed symmetrically with respect to the axis ofthe front unit and the rod portions 321 and 320 are effectively alignedas shown. This is essentially equivalent to a solid, one-piece tie rod,for example the rod 169 as shown in the first embodiment. However, asthe bellcrank rotates about the axis 325, overall or effective length ofthe tie rod assembly decreases which causes the swivel angles throughwhich the front wheels rotate to be less than if the tie rod length wereconstant. Thus, it can be seen that, as the swivel angles of the frontwheels increase with respect to the front unit, the bellcrank rotatesthrough an increasingly larger angle from the symmetrical alignedposition and effective length of the tie rod decreases proportionately.As effective length of the tie rod decreases as the overall angle of thewheels increases, rate of increase of the swivel angles of the frontwheels decreases from the increase which would occur with a one piecetie rod. This overall decrease in swivel angle is referred to as "swivelangle correction" and is necessary to avoid wheel scuffing due toarticulation about the articulation axis 18 as will be described withreference to FIGS. 8-11 and Table 2 following.

Thus, the bellcrank 323 and two rod portions serve as a shorteningstructure for a shortening effective length of the tie rod assembly asthe wheels are swivelled from a straight travel alignment configurationtowards a "hard lock configuration turn". The arms 330 and 331 aredisposed to each other at an angle 333, which is between about 30 and 70degrees. If necessary, to enable easy adjustment of the amount of "tierod shortening" generated by the bellcrank, so as to find the optimumgeometry by experiment, the angle 333 can be made to be variable bysimple mechanical adjustment, e.g. nuts and bolts fitting into alignableopenings of the arms, not shown.

It can be seen that actuation of the articulation steering actuators 236and 237, and the kingpin steering actuators 172 and 173 of the frontwheels are combined or integrated by the primary steering signalapparatus, namely the valve unit 285, and the secondary steering signalapparatus, namely the valves 295 and 296. The valve unit 285 and theproportioning valves 295 and 296 act as a steering integrator to ensurethat a controlled relationship exists between an articulation angle ofthe front unit with respect to the rear unit, and the swivel angles ofthe front wheels with respect to the front unit. This controlledrelationship is described in greater detail with reference to FIG. 11.The wheel 186 provides a manual steering control for controllingactuation of the steering actuator which in turn controls thearticulation angle between the front and rear units, and the respectiveswivel angles of the right hand and left hand front wheels with respectto the front unit. The steering integrator functions with the tie rodshortening structure, that is the bellcrank and the two interconnectedtie rod portions, which ensures that the swivel angles of the frontwheels are corrected in proportion to the articulation angle as will bedescribed with reference to FIGS. 8 through 11.

FIGS. 8 through 11

FIGS. 8 through 11 show operation of main components of the vehicle 10in two modes of steering, thus illustrating main geometricalconsiderations which characterize each mode of steering. In FIGS. 8through 10, the front wheels 22 and 23 are shown undesignated in brokenoutline in straight alignment, and in full outline when swivelled tonegotiate a turn.

FIG. 8 shows a prior art conventional road vehicle with a rigid body, inwhich the front and rear units 13 and 14 are maintained aligned, that isthe articulation joint 16 is made inactive thus eliminating the secondmode of steering. The front wheels 22 and 23 are mounted for kingpinsteering for the first mode of steering and the rear wheels 24 and 25are non-steering wheels, that is they are journalled only for rotationabout wheel axes and thus eliminates the third mode of rear wheelkingpin steering. The front wheels 22 and 23 have swivelled aboutrespective swivel axes and are shown inclined at swivel angles 340 and341 to the straight aligned position of the wheels. If any minorangulation of the wheels due to "toe-in" is ignored, the angles 340 and341 represent swivel angles of the front wheels with respect to the axis19 of the front unit. As is well known, the angle 340 of the right handwheel 22 on the outside of the turn is smaller than the angle 341 of theleft hand wheel 23 on the inside of the turn. The one-piece tie rod 169of FIG. 4 is shown extending between the tie rod arms of the wheels.This represents conventional kingpin or modified Ackerman steering, inwhich axes 342 and 343 of the front wheels 22 and 23 projected inwardlyand intersect at a point of intersection 346. The intersection 346coincides with a projection of coincident rear wheel axes 345 of therear wheels 24 and 25, which, as previously stated, are non-steering.This geometry results in negligible scuffing of the wheels as thevehicle negotiates a turn centred on the intersection 346. This type ofsteering requires a relatively large space as the rear wheels tend to"cut-off" the corner and do not follow the same arc traced out by thefront wheels as the vehicles negotiates the turn, thus requiring morespace on the inside of the turn.

Referring to FIG. 9, the axis 19 of the front unit is shown inclined atan articulation angle 348 to the axis 20 of the rear unit byarticulating about the articulation joint 16 towards the left. Becausethe tie rod 169 has a fixed length, the wheels 22 and 23 are at the sameangles 340 and 341 respectively relative to the front unit as shown inFIG. 8. In other words, there has been no compensation for the swivelangle of the wheels at this stage, which causes the point ofintersection 346 of the front wheel axes, shown as 342.1 and 343.1, tomove rearwardly to a displaced position shown at 346.1 having swungthrough an angle 350 from the point 346, which equals the angle 348.With no correction of the swivel angle of the front wheels, scuffingwould occur when the vehicle negotiates a curve. In the first embodimentof the invention as shown in FIGS. 1 through 6, this scuffing can beavoided by independently manually adjusting angle of the steering wheelby rotating it clockwise so as to reduce the swivel angles of the frontwheels as will be described with reference to FIG. 10. In FIG. 9, it canbe seen that radius of the turn, designated 347.1, has been reduced by aspacing 353, which is relatively insignificant.

Referring to FIG. 10, the front and rear units are shown inclined at theangle 348, similarly to FIG. 9, but are provided with the alternativetie rod assembly 287 of FIG. 7 which provides a "correction" to theswivel angles 340 and 341 of FIG. 9. Thus, the wheels 22 and 23 areshown inclined at "corrected" swivel angles 356 and 357 respectively inwhich the angle 356 is less than the angle 340, and the angle 357 isless than the angle 341. In these new positions, axes 342.2 and 343.2 ofthe wheels 22 and 23 now intersect at an intersection 346.3 which isagain located on the projected rear wheel axis 345 and thus shouldproduce negligible scuffing of the wheels during the turn. The newposition 346.3 is attained by correcting one or both of the swivelangles 340 or 341 to attain the new swivel angles 357 and/or 356. Thus,both angles 340 and 341 could be corrected by relatively small amountsso as to reposition the intersection of the axes 343.2 and 342.2 on theaxis 345. Alternatively, one of the wheels could remain at the sameinclination as found in FIG. 9, and the other wheel could be correctedthrough a greater angle so that the axis of the other wheel intersectsthe intersection of the first wheel and the rear wheel axis 345.

Whether correcting one or both swivel angles, the intersection 346.3 isspaced inwardly from the intersection 346 by a spacing 353 which resultsin a much smaller radius of turn, designated 347.2. Thus, in someinstances the radius of turn of the corrected, two mode steering shownin FIG. 10 is approximately 60-70 per cent of the radius of turn 347 ofthe normal single mode kingpin steering of FIG. 8. Thus, in FIG. 10,intersection of the axes 342.2 and 343.2 is adjusted to intersect on theaxis 345 by changing inclination of either one or both of the frontwheels to intersect the axis 345 at a position closely adjacentintersection of one or both of the axes as shown in FIG. 9. This resultsin a minor correction of at least one of the swivel angles of the frontwheels to attain the intersection 346.3, which results in a shorterradius of turn with less wheel scuffing than that shown in FIG. 9. Theswivel angle correction is attained automatically by effective"shortening" of the tie rod assembly by use of the bellcrank as shown inFIG. 7.

It is noted that the mode of steering of the second embodiment 280 asshown in FIG. 10 cannot be attained manually when operating the firstembodiment of FIGS. 1 through 6. In the first embodiment, the operatorcannot manually adjust the steering wheel to correct for the effect ofthe articulation angle on the swivel angles of the front wheel kingpinsteering as the tie rod 169 (FIG. 4) has a fixed length. This tends toproduce wheel scuffing as described which is also a problem for theprior art vehicle fitted with two mode steering, as previouslydiscussed. In any event, attempting to correct front wheel steering incombination with articulation steering is difficult for a noviceoperator.

In the following discussion, the left front wheel is closest to theintersection 346.3 of FIG. 10, i.e. the center of the turn, and istermed the "inside" wheel, and the right front wheel is thus the"outside" wheel. Also, for convenience of explanation, the inside wheelis taken as a datum wheel and responds directly to turns of the steeringcolumn, i.e. it is not "corrected", and therefore the swivel angle ofonly the outside wheel is "corrected" to maintain intersection of thewheel axes at 346.3. To simplify design, a "steering angle ratio" isselected, and is defined as ratio of the articulation angle 348 to theangle 357 of the inside (or left) wheel, both angles being defined withrespect to straight ahead positions as seen in FIG. 10. Preferably, thesteering angle ratio is fixed, i.e. is constant for a particularvehicle, to simplify design considerations.

Referring to FIG. 11 for a more detailed analysis, steering componentsrelating to the front wheels are shown in full outline in a straightahead position, and in broken outline in a hard lock turn to the leftper arrow 351, which corresponds to the positions shown in the diagramof FIG. 10. Undesignated arrows show fluid flow directions in hydrauliclines and corresponding movements of the piston rods of the steeringcylinders 172 and 173 following initiation of the turn to the left. Itcan be seen that as the left hand steering cylinder 173 extends, theright hand cylinder 172 retracts, which causes anti-clockwise rotationof the respective tie rod arms 160 and 165 and actuator arms 162 and 166per undesignated arrows. This causes the right and left hand tie rodportions 320 and 321 to shift generally longitudinally to the right.Movement of the inner ends of the tie rods is controlled by the steeringbellcrank 323 which rotates about the bellcrank axis 325 in response tomovement of the tie rod portions. As discussed previously, the right andleft hand arms 330 and 331 of the bellcrank 323 are inclined to eachother at the angle 333, and when the bellcrank rotates through a fullleft lock angle 354, the rods assume extreme positions as shown inbroken outline, and the shift angle between the arms is designated in aswivelled position 333.1. The right hand and left hand tie rod arms 160and 165 are shown to have swung through angles 356 and 357 respectivelywhich corresponds to the angles through which the wheels swing as shownin FIG. 10.

For the turn to the left as shown in FIG. 11, fluid flow to and from theright hand and left hand articulation cylinders 236 and 237 respectively(see FIG. 7) is shown partially passing along fragmented second conduits306 and 316 respectively, resulting in corresponding relative rotationbetween the front and rear units 13 and 14 about the articulation joint16 per FIG. 10.

In summary, ratio of relative rotation through an articulation anglebetween the front and rear units about the articulation axis, andrelative rotation of one front wheel through a swivel angle about arespective swivel axis relative to the front unit is defined by thesteering angle ratio as determined by the controlled relationshipassociated with the steering integrator.

Clearly, many factors must be considered in overall design of theintegrated steering as discussed above, and the relative sizes andpositions of components in the schematics of the FIGS. 7, 10 and 11could be changed to achieve particular steering angle ratios andsteering fluid ratios. Tables 1 and 2 following are based on atheoretical model, which for simplicity has a constant steering angleratio and a constant steering fluid ratio. Table 1 shows one example ofmain physical characterises of particular portions of a sample vehiclewhich is not necessarily compatible with the proportions of FIGS. 7, 10and 11. Table 2 also relates to that particular sample and showscorresponding relationships between four incremental rotationalpositions of the steering column 187 and corresponding resulting anglesthrough which the front wheels and bellcrank swivel about respectiveaxes, and the front and rear units swivel about the articulation pin 16.The right hand column of Table 2 shows percentage of effectiveshortening of the tie rods due to corresponding angle of rotation of thebellcrank.

    ______________________________________    NAME         DEFINITION         SIZE    ______________________________________    Wheelbase    longitudinal spacing between front                                    140 inches                 and rear axles when the vehicle                                    (3556                 units are aligned  mm)    Wheel track  transverse spacing between central                                    861/2                 diametrical planes of wheels of a                                    inches                 pair               (2220                                    mm)    Distance between                 transverse spacing between right                                    60 inches    kingpins     and left swivel axes 154 and 155                                    (1524                                    mm)    Steering Bellcrank angle                 Angle 333 between arms 331 and                                    42 degrees                 332    Steering angle ratio                 Ratio of steering angle changes -                                    1.5:1.0                 articulation steering (Angle 348) to                 kingpin steering (Angle 357)    Steering fluid ratio                 Ratio of fluid volume flows -                                    4.15:1.0                 articulation cylinders (236,237) to                 kingpin cylinders (172, 173)    ______________________________________

                  TABLE 2    ______________________________________    ANGULAR RELATIONSHIPS                                ARTICULA-             INSIDE             TION    ANGLE OF WHEEL    OUTSIDE   ANGLE 348                                        EFFECTIVE    STEERING ANGLE    WHEEL     BETWEEN SHORTENING    COLUMN   357      ANGLE 356 UNITS   OF TIE ROD    ______________________________________    quarter  3 degrees                      2.84 degrees                                4.5 degrees                                        0.04 per cent    lock    half     6 degrees                      5.38 degrees                                9.0 degrees                                        0.16 per ccnt    lock    three quarter             9 degrees                      7.66 degrees                                13.5 degrees                                        0.35 per cent    lock    full     12 degrees                      9.72 degrees                                18.0 degrees                                        0.59 per cent    lock    ______________________________________

Clearly, wide variations from the above are possible depending onparticular vehicle design and requirements. In general, the greater thesteering angle ratio (i.e. articulation steering to king pin steering),the greater is the need to modify the steering angle generated by thebellcrank, and thus the greater the angle 333 between the arms of thebellcrank. In addition, it can be seen from the above table thateffective shortening of the tie-rod is relatively small, and that anyfluid displaced at relatively low pressure from one of the cylinders172, 173, 236 or 237 which is not absorbed by the remaining cylinder ofthe pair is accommodated by dumping excess fluid into the sump. Clearly,relative angular relationship between the two front steering wheels iscontrolled by the steering bellcrank, relative angles of the tie-rodsand geometry of other steering components as is well known.

FIGS. 10 and 11 represent integrated steering, i.e. the automaticcombination of two modes of steering, namely front wheel kingpinsteering in combination with articulation steering, with the operatorcontrolling the steering wheel and axes of the rear wheels beingstationary with respect to the rear unit. Clearly, both embodimentsshown in FIGS. 4 and 7 are capable of rear wheel kingpin steering also,which would provide three modes of steering in which the rear wheels areswivelled with respect to the rear unit as shown in FIG. 5. As shown inFIG. 5, in three mode steering, axes of the rear wheels would intersectthe axes of the front wheels and an axis passing through thearticulation axis as shown in FIG. 5.

From the above it can be seen that the swivel angle correction for atleast one front wheel due to the tie rod shortening effect of thebellcrank must be compatible with the output signal ratio between thefirst and second outputs of the valves 295 and 296 and other geometry.As seen in Table 2, for relatively shallow turns, the adjustment isfairly insignificant, but for relatively tight turns the adjustmentbecomes fairly significant.

FIG. 12

In the embodiments of FIGS. 4 and 7, the kingpin steering assemblies ofthe front wheels include transversely extending kingpin steeringcylinders 172 and 173 which are located under the vehicle, forward ofthe front axle, and generally aligned with the respective actuator arms162 and 166. Because the front cylinders are located in front of thefront axle, they are susceptible to damage from tree stumps or bouldersthat may be obscured from the operator's view.

A third embodiment 361 of the invention relocates the kingpin steeringcylinders of the previous embodiments to a less vulnerable location, andeliminates the requirement for the actuator arms 162 and 166. A vehicleincorporating the third embodiment 361 can be essentially identical tothe vehicle 280 of FIG. 7, and thus is not shown fully or described indetail except where there are differences as follows. The thirdembodiment 361 includes an alternative tie rod assembly 362 whichcomprises an alternative bellcrank 363 which cooperates with thesteering assemblies of the front wheels generally similarly to that asshown in FIG. 7. Thus, the right hand and left hand wheels 22 and 23have respective tie rod arms 160 and 165 which are coupled to therespective tie rod portions 320 and 321 as shown. The alternativebellcrank 363 has right hand and left hand arms 364 and 365 which arecoupled to the inner ends of respective tie rod portions 320 and 321. Asbefore, the bellcrank 363 is journalled for rotation about an axis 367and serves as a shortening structure to "shorten" the tie rod as thewheels swivel from the straight aligned position.

The alternative steering apparatus further includes right hand and lefthand kingpin steering cylinders 370 and 371 which function equivalentlyto the cylinders 172 and 173 of FIGS. 4 and 7, but cooperate with thewheels through the bellcrank 364. Thus, the steering cylinder 370extends between the right hand arm 364 of the bellcrank and a suitableportion of the front unit to apply force to the bellcrank to swivel thefront wheels. Similarly, the left hand cylinder 371 extends between theleft hand arm 365 and the front unit, and thus the cylinders 370 and 371extend and retract simultaneously working in concert to swivel the frontwheels. The cylinders 370 and 371 receive fluid as before from the powersteering valve and pump unit 285 which proportions fluid to thecylinders 370 and 371, and also to the articulation steering cylinders236 and 237 (shown in FIG. 7). Apart from changing the proportion offluid between the kingpin steering cylinders and the articulationsteering cylinders, other aspects of the alternative third embodiment361 are generally similar to the alternative embodiment 282 of FIG. 7.Clearly, the steering cylinders 370 and 371 can be located within arecess of the front unit and cooperate with the bellcrank in arelatively safe location when compared with the exposed location forwardof the front axle as shown in FIG. 7. In this arrangement, it can beseen that the front kingpin steering actuator, which can also be asingle linear actuator, or a rotary actuator which cooperates with thetie rod shortening structure of the alternative tie rod assembly 362,i.e. the bellcrank 363, to swivel the front wheels.

Clearly, the fixed relationship between the front wheel swivel anglesand the articulation angles of the second and third embodiments 280 and361 simplify operation of the vehicles, but limit their versatilitysomewhat as the two steering modes are not independent of each other asin the first embodiment 10.

We claim:
 1. A wheeled vehicle (10) comprising: front and rear units(13,14), each unit having right hand (22, 24) and left hand (23, 25)wheels as a pair, the units being connected together to permit relativerotation therebetween about a generally vertical articulation axis (16),the front wheels (22, 23) being mounted on the front unit (13) forkingpin steering to permit rotation of each front wheel about arespective generally vertical swivel axis (154, 155) relative to thefront unit; an articulation steering actuator (236, 237) cooperatingwith the front and rear units (13, 14) to cause the said relativerotation to effect articulation steering as required; a front kingpinsteering actuator (172, 173) cooperating with the front wheels (22, 23)to cause the rotation about the swivel axes (154, 155) to effect thesaid kingpin steering between the wheels of the front unit as required,the steering actuators being hydraulically actuated; a manual steeringcontrol (225) for controlling actuation of the articulation steeringactuator (236, 237) between the front and rear units (13, 14), and amanual steering control (186) for controlling actuation or the frontkingpin steering actuator (172, 173); the vehicle being characterizedby:(a) the rear wheels (24, 25) being mounted on the rear unit (14) forkingpin steering to permit rotation of each rear wheel about arespective generally vertical swivel axis (208, 209) relative to therear unit, (b) a rear kingpin steering actuator (212, 213) thereofcooperating with the rear wheels to cause rotation about the respectiveswivel axes to effect kingpin steering between the wheels of the rearunit, and (c) a manual steering control (216) or controlling actuationof the rear kingpin steering actuator.
 2. A vehicle as claimed in claim1, further characterized by:(a) the manual steering control for thefront kingpin steering actuators (172, 173) comprising a steering column(186) rotatable by the operator, the steering column controlling a valve(195) which controls swivel angle (340, 341) of the front wheels (23,24) with respect to the front unit, and (b) the manual steering control(216) for the rear kingpin steering actuators (212, 213) comprising arear steering actuator unit which is a directional control valve 216)which controls swivel angle of the rear wheels (24, 25) with respect tothe rear unit.
 3. A vehicle as claimed in claim 1, further characterizedby:(a) the manual steering control for the articulation steeringactuators (236, 237) is a control valve (225) for controlling anarticulation angle (348) between the front and rear units (13, 14).
 4. Avehicle as claimed in claim 2, further characterized by:(a) the frontkingpin steering control (189) and the rear kingpin (216) steeringcontrol being operable independently of each other.
 5. A vehicle asclaimed in claim 3, further characterized by:(a) the articulationsteering control (225) is operable independently of the manual controls(186, 216) for controlling actuation of the front and rear kingpinsteering actuators with respect to both units (13, 14).
 6. A wheeledvehicle comprising:(a) front and rear units, each unit having right handand left hand wheels as a pair, the units being connected together topermit relative rotation therebetween about a generally verticalarticulation axis, the front wheels being mounted on the front unit forkingpin steering to permit rotation of each front wheel about arespective generally vertical swivel axis relative to the front unit,each front wheel having a steering assembly which includes a respectivefront tie rod arm extending therefrom, and the front unit including atie rod assembly connecting together the tie rod arms, the tie rodassembly including a tie rod shortening structure for shorteningeffective length of the tie rod assembly as the wheels are swivelledfrom straight line alignment, (b) an articulation steering actuatorcooperating with the front and rear units to cause the said relativerotation to effect articulation steering as required, (c) a frontkingpin steering actuator cooperating with the front wheels to cause therotation about the swivel axes to effect the said kingpin steeringbetween the wheels of the front unit as required, (d) a steeringintegrator cooperating with the articulation steering actuator and thekingpin steering actuator to automatically integrate articulationsteering between the front and rear units and kingpin steering of thefront wheels, so that a controlled relationship exists between anarticulation angle of the front unit with respect to the rear unit, andswivel angles of the front wheels with respect to the front unit, and(e) a manual steering control for controlling actuation of the steeringintegrator which in turn controls an articulation angle between thefront and rear units, and respective swivel angles of the right hand andleft hand front wheels with respect to the front unit.
 7. A wheeledvehicle as claimed in claim 6, in which the steering integrator isfurther characterized by:(a) a primary steering signal apparatus (285)responsive to the manual steering control (186), the primary steeringapparatus having a primary signal output (291, 292), and (b) a secondarysteering signal apparatus (295, 296) having an input (297, 298)connectable to the output (291, 292) of the primary steering signalapparatus and first and second outputs (299, 305, 308, 312), the firstoutput (305, 312) being transmitted to the articulation steeringactuator (236, 237), and the second output (299, 308) being transmittedto the front kingpin steering actuator (172, 173; 370, 371), the firstand second outputs of the secondary steering signal apparatus having anoutput signal ratio which reflects the controlled proportionalrelationship between the articulation angle (348) of the front unit (13)with respect to the rear unit (14), and the swivel angles (356, 357) ofthe front wheels (22, 23) with respect to the front unit (13).
 8. Avehicle as claimed in claim 6, in which the steering integrator isfurther characterized by:(a) a primary steering signal apparatus (235)responsive to the manual steering control (136), the primary signalsteering apparatus having first and second primary signal ports (291,292) and (b) a secondary steering signal apparatus comprising first andsecond proportioning units (295, 296) having first and second combinedports (297, 298) communicating with the first and second primary signalports (291, 292) respectively of the primary steering signal apparatus(285), each proportioning unit also having a front split port (299, 308)and a rear split port (305, 312), the front split port of eachproportioning unit communicating with the front kingpin steeringactuator (172, 173; 370, 371), and the rear split port of eachproportioning unit communicating with the articulation steering actuator(236, 237).
 9. A vehicle as claimed in claim 8, further characterized inthat:(a) the front kingpin steering actuator is at least one hydraulickingpin steering cylinder (171, 172; 371, 372) having an extension port(177, 181) and an retraction port (178, 182), in which the kingpinsteering cylinder extends or retracts when positive pressure is appliedto the extension or retraction port respectively, (b) the articulationsteering actuator is at least one hydraulic articulation steeringcylinder (236, 237) having an extension port (314, 317) and a retractionport (315, 318), in which the articulation steering cylinder extends orretracts when positive pressure is applied to the extension orretraction port respectively, and (c) a plurality of hydraulic conduits(301, 302, 306, 310, 311, 316) hoses extending between the secondarysteering signal apparatus (295, 296) and the front kingpin steeringcylinder (171, 172; 371, 372) and the articulation steering cylinder(236, 237), such that fluid passing between the secondary steeringsignal apparatus and the steering cylinders results in essentiallysimultaneous actuation of the kingpin steering cylinder and thearticulation steering cylinder.
 10. A vehicle as claimed in claim 7,further characterized in that:(a) the output signal ratio of thesecondary steering signal apparatus (295, 296) is equal to a steeringfluid ratio defined by fluid volume flow with respect to thearticulation steering actuator (236, 237) and fluid volume flow withrespect to the front kingpin steering actuator (172, 173; 370, 371). 11.A vehicle as claimed in claim 8, further characterized in that:(a) ratioof total volume flow with respect to the rear split ports (305, 312) andtotal volume flow with respect to the front split ports (299, 308)defines a steering fluid ratio, which controls relative actuation of thearticulation steering actuator (236, 237) and the kingpin steeringactuator (172, 173; 371, 370).
 12. A vehicle as claimed in claim 10further characterized in that:(a) the steering fluid ratio isessentially constant for all angles of a turn.
 13. A vehicle as claimedin claim 6, further characterized by:(a) the front kingpin steeringactuator (172, 173; 370, 371) cooperating with the front tie rodassembly (287, 362) to apply force to the tie rod assembly to swivel thewheels.
 14. A vehicle as claimed in claim 13, further characterizedby:(a) the steering actuator (370, 371) cooperating with the tie rodshortening structure (363) to swivel the wheels.
 15. A wheeled vehiclecomprising:(a) front and rear units, each unit having right hand andleft hand wheels as a pair, the units being connected together to permitrelative rotation therebetween about a generally vertical articulationaxis, the front wheels being mounted on the front unit for kingpinsteering to permit rotation of each front wheel about a respectivegenerally vertical swivel axis relative to the front unit, (b) anarticulation steering actuator cooperating with the front and rear unitsto cause the said relative rotation to effect articulation steering asrequired, (c) a front kingpin steering actuator cooperating with thefront wheels to cause the rotation about the swivel axes to effect thesaid kingpin steering between the wheels of the front unit as required,(d) a steering integrator cooperating with the articulation steeringactuator and the kingpin steering actuator to automatically integratearticulation steering between the front and rear units and kingpinsteering of the front wheels, so that a controlled relationship existsbetween an articulation angle of the front unit with respect to the rearunit, and swivel angles of the front wheels with respect to the frontunit in which ratio of relative rotation through an articulation anglebetween the front and rear units about the articulation axis, andrelative rotation of one front wheel through a swivel angle about arespective swivel axis relative to the front unit is defined by asteering angle ratio as determined by the controlled relationshipassociated with the steering integrator, and (e) a manual steeringcontrol for controlling actuation of the steering integrator which inturn controls an articulation angle between the front and rear units,and respective swivel angles of the right hand and left hand frontwheels with respect to the front unit.
 16. A vehicle as claimed in claim15, further characterized by:(a) the steering angle ratio beingessentially constant for all angles of a turn.
 17. A wheeled vehicle asclaimed in claim 15, in which the steering integrator comprises:(a) aprimary steering signal apparatus responsive to the manual steeringcontrol, the primary steering apparatus having a primary signal output,and (b) a secondary steering signal apparatus having an inputconnectable to the output of the primary steering signal apparatus andfirst and second outputs, the first output being transmitted to thearticulation steering actuator, and the second output being transmittedto the front kingpin steering actuator, the first and second outputs ofthe secondary steering signal apparatus having an output signal ratiowhich reflects the controlled proportional relationship between theangle of the front unit with respect to the rear unit, and the swivelangles of the front wheels with respect to the front unit.
 18. A vehicleas claimed in claim 15 in which the steering integrator comprises:(a) aprimary steering signal apparatus responsive to the manual steeringcontrol, the primary steering signal apparatus having first and secondprimary signal ports, and (b) a secondary steering signal apparatuscomprising first and second proportioning units having first and secondcombined ports communicating with the first and second primary signalports respectively of the primary steering signal apparatus, eachproportioning unit also having a front split port and a rear split port,the front split port of each proportioning unit communicating with thefront kingpin steering actuator, and the rear split port of eachproportioning unit communicating with the articulation steeringactuator.
 19. A vehicle as claimed in claim 18, in which:(a) the frontkingpin steering actuator is at least one hydraulic kingpin steeringcylinder having an extension port and a retraction port, in which thekingpin steering cylinder extends or retracts when positive pressure isapplied to the extension or retraction port respectively. (b) thearticulation steering actuator is at least one hydraulic articulationsteering cylinder having an extension port and a retraction port, inwhich the articulation steering cylinder extends or retracts whenpositive pressure is applied to the extension or retraction portrespectively, and (c) a plurality of hydraulic hoses extending betweenthe secondary steering signal apparatus and the front kingpin steeringactuator and the articulation steering actuator, such that fluid passingbetween the secondary steering signal apparatus and the steeringcylinders results in essentially simultaneous actuation of the kingpinsteering cylinder and the articulation steering cylinder.
 20. A vehicleas claimed in claim 17, in which:(a) the output signal ratio of thesecondary steering signal apparatus is equal to a steering fluid ratiodefined by fluid volume flow with respect to the articulation steeringactuator and fluid volume flow with respect to the front kingpinsteering actuator.
 21. A vehicle as claimed in claim 18, in which:(a)ratio of total volume flow with respect to the rear split ports andtotal volume flow with respect to the front split ports defines asteering fluid ratio, which controls relative actuation of thearticulation steering actuator and the kingpin steering actuator.
 22. Avehicle as claimed in claim 20, in which:(a) the steering fluid ratio isessentially constant for all angles of a turn.
 23. A wheeled vehiclecomprising:(a) front and rear units, each unit having right hand andleft hand wheels as a pair, the units being connected together to permitrelative rotation therebetween about a generally vertical articulationaxis, the front wheels being mounted on the front unit for kingpinsteering to permit rotation of each front wheel about a respectivegenerally vertical swivel axis relative to the front unit, in which eachfront wheel has a steering assembly which includes a respective fronttie rod arm extending therefrom, the tie rod arms having respective armaxes inclined equally to a longitudinal front unit axis when the wheelsare straight aligned, such that the arm axes of the front wheelsprojected rearwardly intersect a rear wheel axis, the front unit alsohaving a front tie rod interconnecting the tie rod arms together tocoordinate swivelling of the front wheels; and in which each rear wheelhas a steering assembly which includes a respective rear tie rod armextending therefrom, the rear tie rod arms having respective arm axesinclined equally to a longitudinal vehicle axis when the wheels arestraight aligned such that the arm axes of the rear wheels projectedforwardly intersect a front wheel axis, the rear unit also having a reartie rod interconnecting the tie rod arms of the rear wheels together tocoordinate swivelling of the rear wheels, (b) an articulation steeringactuator cooperating with the front and rear units to cause the saidrelative rotation to effect articulation steering as required, (c) afront kingpin steering actuator cooperating with the front wheels tocause the rotation about the swivel axes to effect the said kingpinsteering between the wheels of the front unit as required, (d) asteering integrator cooperating with the articulation steering actuatorand the kingpin steering actuator to automatically integratearticulation steering between the front and rear units and kingpinsteering of the front wheels, so that a controlled relationship existsbetween an articulation angle of the front unit with respect to the rearunit, and swivel angles of the front wheels with respect to the frontunit, and (e) a manual steering control for controlling actuation of thesteering integrator which in turn controls an articulation angle betweenthe front and rear units, and respective swivel angles of the right handand left hand front wheels with respect to the front unit.
 24. A vehicleas claimed in claim 23, in which the tie rod shortening structure of thetie rod assembly is characterized by:(a) right hand and left hand tierod portions (320, 321), each rod portion having respective outer andinner ends (327, 328), the outer ends of the right hand and left handtie rod portions being connected to the tie rod arms (160, 165) of theright hand and left hand wheel steering assemblies respectively, and (b)a steering bellcrank (323, 363) interconnecting inner ends (328) of theright hand and left hand tie rod portions (320, 321), the bellcrankbeing journalled for rotation in response to movement of the wheels (22,23), so that as the swivel angles (356, 357) of the front wheels (22,23) with respect to the front unit (13) increases, effective length ofthe tie rod decreases.
 25. A vehicle as claimed in claim 23, furthercharacterized by:(a) the front tie rod assembly (287) including a tierod shortening structure (323, 363) for shortening effective length ofthe tie rod assembly as the front wheels (23, 24) are swivelled from thestraight line travel alignment.
 26. A vehicle as claimed in claim 25, inwhich the tie rod shortening structure of the tie rod assemblycomprises:(a) right hand and left hand tie rod portions, each rodportion having respective outer and inner ends, the outer ends of theright hand and left hand tie rod portions being connected to the tie rodarms of the right hand and left hand wheel steering assembliesrespectively, and (b) a steering bellcrank interconnecting inner ends ofthe right hand and left hand tie rod portions, the bellcrank beingjournalled for rotation in response to movement of the wheels, so thatas the swivel angle of the front wheel with respect to the front unitincreases, effective length of the tie rod decreases.
 27. A vehicle asclaimed in claim 24, in which:(a) the front kingpin steering actuatorcomprises a steering cylinder (370, 371) extending between the bellcrank(363) and the front unit (13) to apply force to the bellcrank to swivelthe front wheels.